Subdivision work that converted a balcony into a self-contained flat was one of the factors that triggered a partial collapse of a 61-year-old tenement block in Kowloon, according to an investigation by the Buildings Department.

The authority has not ruled out taking legal action if they find evidence that the conversions, which contributed to a 44 per cent increase in weight, violated building regulations.

No one was injured, but all of its residents – at least 22 of them – had to move out of the six-storey building due to structural safety concerns.

The collapse exposed the interior of the subdivided unit, with a bunk bed dangling precariously from the edge, while furniture and other household items were thrown onto the street.

The department’s investigation report, released on Wednesday, concluded that there were four main factors that contributed to the collapse: declined concrete strength, corrosion of steel reinforcement bars, increase in loading and a lack of maintenance.

An extra two inches of floor finishing was added to level off the balcony with the interior floor, while almost three inches of additional cement for wall rendering and windows added to the weight of the flat.

Kenny Tse Chi-kin, vice-chairman of the Hong Kong Institute of Surveyors’ building surveying division, said that a 44 per cent increase in dead load was “significant”.

“It can become dangerous when landlords or tenants do not find authorised personnel to help them calculate whether the renovation work would exceed the maximum loading designed for the building,” Tse said.

The report added that the concrete strength had declined by 53 per cent compared to its original design, while cracks in the walls and rusty reinforcement bars showed that the building had not been properly maintained.

The incident reignited concerns over the structural safety of decaying walk-up flats in Hong Kong, where there are some 6,000 buildings which are at least half a century old.

While the investigation showed that there were clear signs of overloading, authorities may run into difficulties with prosecuting those responsible, said City University associate professor Simon Yau Yung.

“It can be difficult for the department to try to identify whether it was the landlord or the tenant who carried out conversion works at what time. For example, if the landlord can prove with photos that the flat was not touched prior to leasing it out, but they might not have such evidence,” said Yau, who specialises in unauthorised building work research.

The department requested that owners submit remedial proposals and to execute repair works.

Henderson Land, one of the major owners of the building, confirmed that they have appointed an authorised person to follow up on the building inspection notices.

A spokeswoman for Henderson Land declined to comment on what kind of proposals to salvage the building would be feasible.

The building is one of dozens in the area acquired by Henderson Land for future redevelopment projects.